[ TERC VHS-1 Sun Photometer ] [ Manual TOC ]

How to Calibrate the TERC VHS-1 Sun Photometer

All Sun photometers determine the clarity of the atmosphere, and thus the blockage of direct sunlight caused by haze, smoke, dust and smog, by measuring the intensity of sunlight. When properly calibrated, different Sun photometers will give nearly the same result.

One way to calibrate a Sun photometer is to illuminate its detector with light from a calibrated light source known as a standard lamp. The problem with this approach is that calibrated lamps cost hundreds of dollars. And they must be powered by very stable power supplies that cost a few thousand dollars.

Fortunately there's a free, highly stable standard lamp that anyone can use. It's the Sun! Although sunlight varies slightly in intensity during the solar cycle, for practical purposes, the Sun can be considered a standard lamp. In fact, for more than a century scientists have used the Sun to calibrate various kinds of Sun photometers.

During the early 1900's, Samuel Langley and Charles Abbot of the Smithsonian Institution perfected the method used to calibrate Sun photometers that is still used today. The method is named for Langley in honor of his pioneering work in this important field.

You can use the Langley method to calibrate a TERC VHS-1 so that its accuracy is as good as a professional instrument costing hundreds of dollars.

Calibration Supplies

Be sure to collect all the necessary supplies BEFORE your calibration session. Otherwise you may get off to a late start. Before beginning, decide how you will measure the Sun's angle at each measurement. You can use the TERC VHS-1 spreadsheet that accompanies this manual. If you don't have access to a computer, you can use one of the methods described in the Appendix.

Here's a challenge: Measure the Sun's angle using the TERC VHS-1 spreadsheet and one or more of the methods described in the Appendix. Then compare your results.

In addition to what you will need to measure the angle of the Sun (see TERC VHS-1 spreadsheet or the Appendix), here's what you will need:

• TERC VHS-1 Sun Photometer (completely assembled and working)
• Digital voltmeter
• Digital watch or clock
• Method for measuring the Sun's angle (see separate section)
• Notebook and a pen or pencil
• Sunglasses, a hat and possibly sun screen
• Linear graph paper and digital calculator with natural logarithm
• (ln) and sine (sin) keys

• Computer with spreadsheet software
• Comfortable, safe place to sit while making measurements
• Cold drinks and snacks

Calibration Procedure

1. Select a day when the sky is blue with few or no clouds.
2. Calibrate your watch against the time signals broadcast by radio station WWV at the frequencies of 5, 10, 15 and 20 MHz.
3. Before each calibration measurement it's important to check the sky to make sure no clouds or cloud haze cover the Sun. Block the Sun with a book or other opaque object held at arm's length and look for nearby clouds or cloud haze.

4. Switch the TERC VHS-1 switch to the ON position, press the Sun port on the instrument against a dark object (black paper or your clothing) and write the number displayed by the voltmeter in the third of four columns in your notebook. This number is known as your instrument's dark signal. You must measure it each time you measure the Sun.
5. Carefully point the VHS-1 at the Sun by centering the bright circle of light that passes through the upper angle bracket directly over the Sun target. While holding the VHS-1 as steady as possible, read the voltage indicated on the voltmeter. The numbers on the voltmeter display will flicker up and down. For best results, pick the highest number that appears when the unit is pointed directly at the Sun. This number is called the Sun signal. Write the Sun signal in the third column in your notebook and the exact time in the first column. Here is a sample notebook entry for the prototype TERC VHS-1 from 17 March 1996.

   
        Time           Dark           Sun         Sky
   
                       Signal         Signal      Condition
   
   
   
   
   
        012:47:40      0.235          3.12        Clear
   
   

6. If you plan to determine the Sun's angle using one of the manual methods described in the Appendix, try to make your Sun angle and VHS-1 measurements as close together as possible. Ideally, one student should make the VHS-1 measurements and another the Sun angle measurements while a third student records the data in the notebook.
7. Space your measurements 15-20 minutes apart when the Sun is high in the sky. As the Sun rises (or descends), make the observations more frequently.

   Figure 5 shows how the Sun's angle over the horizon changes much faster in the early morning and late afternoon than at midday.

8. Before ending your calibration session, review your notes to make sure everything is legible and complete. Do not guess any times you may have forgotten to record! Instead, don't use any data when you don't know the time it was measured. Write a brief summary about the sky conditions during the measurement time. When you are satisfied that your notes are complete, you and any others who helped should sign and date the notebook.